Plain paper is a favored recording medium for storing and transferring human readable information, and the emergence of electronic document processing systems has enhanced the functional utility of plain paper and other types of hardcopy documents by enabling the application of machine readable digital data thereon. This machine readable data enables the hardcopy document to actively interact with such a document processing system in a variety of different ways when the document is scanned into the system by an ordinary input scanner. See, for example, the copending and commonly assigned U.S. patent applications of Frank Zdybel, Jr. et al. and Walter A. L. Johnson et al., which were filed May 30, 1990 on "Hardcopy Lossless Data Storage and Communications for Electronic Document Processing Systems" (D/89190) and on "Form and System Utilizing Encoded Indications for Form Field Processing" (D/90003), respectively.
As a general rule, digital data is recorded by writing two dimensional marks on a recording medium in accordance with a pattern which encodes the data either by the presence or absence of marks at a sequence of spatial locations or by the presence or absence of mark related transitions at such locations. Ordinary magnetic and optical digital data recording conform to this style of encoding.
In a prior method and apparatus for encoding machine readable information on a paper document, as disclosed for example in copending U.S. patent application Ser. No. 07/560,514 filed July 31, 1990 "Self-clocking Glyph Shape Codes", Bloomberg et al, the contents of which are incorporated by reference herein, an encoded pattern is scanned by a CCD input scanner, digitized, and binary thresholded. In binary thresholding methods, all printed pixels with a digitized value greater than the threshold are interpreted as being 0's, and all printed pixels with a digitized value equal to or less than the threshold value are interpreted as 1's. In the above system, the encoding methods thus use binary thresholding of the signal read from the encoded pattern. For example, if a pattern consists of 16 printed pixels for each encoded bit, the digitized CCD output of the scanner is binary thresholded, and then the interpreting algorithm determines whether the 16 pixel cell has an 8 pixel black slash in it. If there is no slash, the pattern is interpreted as encoding a 0. The above system also suggests an encoding scheme wherein slashes of different directions are used to differentiate between a 0 and a 1. In such arrangements, the analog output is digitized and then binary thresholded.
An example of the encoding system of the above system is shown in FIG. 6 wherein each cell of the image is a 4.times.4 matrix of pixels. The encoding of each cell consists in defining a diagonal marking in the cell, in the form of black image of 8 pixels, in a rotationally dependent manner. Accordingly, a cell having such a properly oriented marking corresponds to a 1, whereas a cell with no darkened pixels corresponds to a 0. With this encoding, it is apparent that the illustrated 8 cells (128 pixels) of FIG. 6 corresponds to binary 01111001, or decimal 121.
While the provision of a hardcopy in which pixels of an image may intentionally have more than two gray levels is known, for example in U.S. Pat. Nos. 4,788,598 and 4,790,566, such techniques are not employed in a manner to reduce the area of the hard copy that must be employed for encoding functions.